Multifrequency lasers are receiving increasing attention in lightwave communication systems. Multiplexing several channels with different wavelengths into a single fiber increases the capacity of a fiber transmission line and enables wavelength-dependent add-drop functions with little or no signal regeneration. It is highly desired to provide a transmitter capable of generating a number of wavelengths that can be modulated at high speed, as this not only improves flexibility of the system but also decreases the cost of maintenance.
Wavelength tunable transmitters often employ electroabsorption modulators to modulate wavelengths applying an absorption band edge. Such a system is described, for example, in C. H. Joyner (the inventor herein), M. Zirngibl, and J. C. Centanni, "An 8-Channel Digitally Tunable Transmitter with Electroabsorption Modulated Output by Selective-Area Epitaxy," IEEE Photonics Technology Letters, Vol. 7, No. 9 (September 1995), at 1013-1015, which is hereby incorporated by reference. These modulators exhibit high speed performance and good extinction at selected wavelengths. However, with use of electroabsorption, the depth of modulation is dependent on wavelength and there is little control over chirp associated with modulation. Channel spacings of 100 GHz for a sixteen channel source and single modulator are not attainable, for example, with electroabsorption modulators, because the voltage needed to achieve this level of performance would be prohibitively high or the extinction levels too low at outer channels.
Another approach for providing a wavelength tunable transmitter besides electroabsorption involves use of a Mach-Zehnder interferometric modulator, which employs phase rotation. In this way, the modulation is not dependent on wavelength and further, the chirp of the output pulse may be tailored and even made negative, which may in some cases be desired for long distance transmission applications.
To illustrate, FIG. 1A shows a typical Mach-Zehnder interferometer 10 requiring use of a beam splitter or coupler 12 for dividing the laser beam U.sub.0 into two paths (U.sub.1 and U.sub.2), that are launched into two fibers 14a, 14b. The phase or intensity of the paths (U.sub.1 and U.sub.2) are altered relative to each other which may be accomplished by use of real path length differences or index path differences. FIG. 1A invokes real path length differences in that the length of fiber 14b exceeds that of fiber 14a. FIG. 1B invokes index path differences in that in FIG. 1B, a patch 16 is disposed along the transmission path of U.sub.2 having a refractive index different from that of the substrate or wafer 20; an electric field applied across the patch 16 induces a change in the phase of the signal U.sub.2 relative to U.sub.1. In either case, two different signals U.sub.1 and U.sub.2 are generated which are recombined with a second beam splitter or coupler 22, preferably with low loss. The superposition of the two waves causes destructive or constructive interference necessary to produce output signal U, a modulated signal relative to input U.sub.0.
While the Mach-Zehnder interferometric modulator can be used to provide a tunable transmitter with good extinction over a broad range of wavelengths, it consumes a relatively large amount of chip space. Thus, it would be advantageous to have a wavelength tunable transmitter providing the bandwidth and chirp control features of the Mach-Zehnder interferometric modulator in a lesser amount of space. The invention provides these advantages, and it also provides the advantage of capturing light in the second order Brillouin zones of a waveguide router which in prior applications have been considered unwanted and discarded. Further advantages may appear more fully upon consideration of the description given below.